Just four weeks after President Donald Trump declared victory over the Islamic State group, the jihadists conducted a brutal attack in Syria on Wednesday that highlighted a very different reality on the ground.

Multiple U.S. service members were killed when a suicide bomber blew himself up at a restaurant in the key city of Manbij in Syria's north, the deadliest attack against U.S. forces since they first deployed in Syria in 2015.

The al-Shabaab jihadist group said Wednesday it carried out the deadly attack on a Nairobi hotel and office complex in retaliation for U.S. President Donald Trump's declaration of Jerusalem as Israel's capital, the SITE monitoring group said.

The group said in a statement picked up by SITE that its fighters stormed the DusitD2 complex on instructions by al-Qaida leader Ayman al-Zawahiri.

Foreign troop numbers in Iraq fell by a quarter during 2018, Prime Minister Adel Abdel Mahdi said, as the fallout fizzled from Washington's announcement it was withdrawing from neighbouring Syria.

"In January 2018 there had been almost 11,000 foreign fighters, about 70 percent of them are American, the others are from other countries," Abdel Mahdi told a weekly press briefing on Tuesday evening.